I. Field of the Invention
This invention relates to the field of paper manufacturing and, more particularly, to maintaining the brightness of bleached pulps or paper containing lignin. Maintaining brightness is also known in the industry as retarding or reducing brightness reversion.
II. Description of the Prior Art
Paper or pulps which contain lignin or ligno-cellulose have historically been bleached and continue to be bleached to obtain whiteness. The major drawback of bleached lignin-containing pulps is that they are easily and extensively darkened by light irradiation. This limits their use in various grades of printing papers.
Paper or pulp used in making newsprint is generally not bleached. If it is bleached, it is relatively mildly bleached as compared to higher quality paper. Therefore, such unbleached or mildly bleached pulps have a darker quality than bleached pulps. Newsprint has a relatively high lignin content and, therefore, has a tendency to become even darker when exposed to light. The invention can be used to prevent darkening of all types of lignin-containing pulps including bleached and unbleached pulps, such as newsprint.
The whiteness of paper is estimated by brightness measurements which are based on the reflectance of light having an average wavelength of 457 nm. An Elrepho brightness meter is one type of the instrument used to measure paper brightness. A low brightness (40% Elrepho) indicates brown or dark paper, while 90% Elrepho typifies white paper. Lignin-rich pulps have brightness values in the range of 50-70% Elrepho, depending on the wood species used and the pulping process. These pulps can be bleached to 70-90% brightness using known brighteners such as hydrogen peroxide, sodium borohydride or sodium dithionite. Hydrogen peroxide is normally used when a brightness of more than 70% is required. A problem associated with bleached lignin-rich pulps is that they may darken by as much as 20 Elrepho points when exposed to natural sunlight during exposure over a period of only one day.
Several methods are presently known to decrease brightness reversion in pulps containing lignin. The disadvantage of these methods is that they add significantly to the cost of the paper manufacturing process and are less effective than the invention.
One known method described in European Patent No. 0 280 332 (Agnemo et al.) consists of serial treatments to reduce the carbonyl groups (photosensitizers) contained in the pulp to alcohol groups. In addition, the process includes alkylation of the phenolic hydroxyl groups in the lignin, from which hydrogen atoms are abstracted, by the use of an alkaline propylene oxide. Addition of fluorescent compounds that absorb or reflect the ultraviolet light which would otherwise excite photosensitizer groups is also disclosed.
There has not been any method taught in the prior art in which additives were specifically selected for their ability to donate hydrogen atoms to reactive chemicals present in the bleached pulp upon exposure to light.
Formic acid, which is a formyl compound, has been used as a catalyst in the brightening process of organic materials (i.e., U.S. Pat. No. 3,723,425, Ackermann et al.). However, formic acid should not be used to retard brightness reversion because of its acidity which is known to accelerate brightness reversion due to light irradiation (Lee et al., J. Amer. Inst. Conserv. 28:1 (1989)).
Of the known processes for maintaining brightness or reducing brightness reversion, none have the capacity to reduce brightness reversion as effectively as the invention, while at the same time minimizing the additional cost of production of the paper. Of the known processes, none is available at a low cost and none has outstanding whiteness stabilization qualities. A need thus persists for an efficient and low cost paper manufacturing process which reduces the brightness reversion of bleached pulps containing lignin.